Magnetite nanoparticles have been successfully used for removal and immobilization of contaminants in water, yet their application in soils combined with in situ magnetic separation remains unexplored. We evaluated the effectiveness and optimal conditions for using magnetite nanoparticles combined with magnetic separation to remove metal(loid)s from contaminated mine soils. Soil samples were incubated (15, 45 days) with varying doses of magnetite (0, 25, 50 g kg⁻¹) and moisture (dry, field capacity) and separated using electromagnet or permanent magnet. This technique achieved up to 44 % As, 65 % Cd, 60 % Cu, 47 % Fe, 40 % Mn, 65 % Pb, and 62 % Zn removal, leaving minimal residual magnetite in the soil. These high removal efficiencies were attributed to the nanoparticles' magnetic properties, adsorption capacity and ability to form aggregates with soil particles. Optimal conditions were 25 g kg⁻¹ of magnetite incubated for 45 days at field capacity and separated by the electromagnet. Higher doses (50 g kg⁻¹) offered minimal improvement at increased costs. The combined use of magnetite nanoparticles and in situ magnetic separation demonstrated a low-impact and cost-effective method for reducing metal(loid) concentrations to levels that facilitate subsequent soil remediation strategies.

The application of organic amendments to contaminated agricultural lands can immobilize metals and improve soil conditions. The chemical structures and long-term stability of commercial humic substances and other composted organic amendments (sheep and horse manure, vermicompost, pine bark, and pruning waste) were analyzed using ¹³C CPMAS NMR, FT-IR, and DSC to evaluate their use in soil remediation. The interactions of humic substances and manure with Cu (0 and 5000 mg kg⁻¹) at different pH (2.5 and 5.0) were studied through a batch adsorption experiment observing the changes in their molecular structure using spectroscopic techniques. Humic substances exhibited high aromaticity and phenolic and carboxylic group content, with great affinity for Cu complexation. Humic substances and pruning waste were the most stable due to their high recalcitrant organic matter contents, whereas manure was the least stable, given the labile nature of its organic matter content. There were considerable changes in the carboxylic and phenolic groups of humic substances with pH, and also with Cu, albeit in a lesser extent, especially at pH 5.0, suggesting the great sorption capacity of humic substances and the key role of pH and these functional groups in metal complexation. Manure did not exhibit such changes. Commercial humic substances could be useful amendments for the remediation of contaminated agricultural soils due to their high sorption capacities and long-term stability.

We studied the effects of commercial humic substances derived from leonardite at different rates (0, 0.25, 2, 10 g kg⁻¹) and pH (4.5, 6.0, 8.0) on Cu and Zn mobility, to evaluate their use for remediation of metal contaminated mine soils and to optimize their application conditions. We conducted a single-step extraction experiment and analyzed extracts for metal concentrations, soluble organic carbon and their E4/E6 ratio (ratio of absorption at 465 to 665 nm). Metal speciation in a soil solution was simulated by the non-ideal competitive adsorption-Donnan (NICA-Donnan) model. Increasing the amount of humic substances and the pH caused higher release rates of soluble organic carbon with a lower humic/fulvic acids ratio. This led to a higher mobility of metals (up to 110 times Cu concentration in control and 12 times for Zn) due to the formation of soluble metal-humic complexes. Speciation modeling predicted that increasing rates of humic substances would result in a higher proportion of Cu and Zn associated with fulvic acids, more mobile than the humic acids fraction. Application of commercial leonardite humic substances at 2–10 g kg⁻¹ and with pH levels similar to or below natural soil could be useful for assisted-phytoextraction of contaminated anthropogenic soils. High rates of humic substances in more alkaline conditions could entail a considerable risk of metal leaching to groundwater, toxicity and transfer to the trophic chain.